1. Field of the Invention
This invention relates to a magnetic carrier for use in an electrographic developing method which effects development of an electrostatic latent image with a magnetic developer at a developing region by rotating a developing sleeve incorporating a magnet therein, thereby causing the aforementioned magnetic developer composed of an insulating toner and a magnetic carrier to be transported to the aforementioned developing region.
2. Description of the Prior Art
A developing method which comprises forming on the surface of a developing sleeve incorporating therein a magnet a magnetic brush of a magnetic developer composed of an insulating toner and a magnetic carrier, and making the magnetic brush come in rubbing contact with the surface of a carrier bearing an electrostatic latent image, thereby developing the electrostaic latent image formed on the surface of the aforementioned carrier to render the image visible. As a popular magnetic developer for the aforementioned developing method, the magnetic developer composed of a magnetic carrier of iron particles having an average particle diameter of 100 to 200 .mu.m and an insulating toner of particles having an average particle diameter of 10 to 20 .mu.m has been used. In this developer, however, the magnetic attraction exerted upon the carrier particles during the formation of a magnetic brush is so strong that the bristles of the magnetic brush are rigidified and the carrier particles are aggregated in the form of chains or fins on the developing sleeve, to induce occurrence of white streaks in solid developed images and other similar troubles. Further the electric resistance of the carrier itself is generally so small as to fall below 10.sup.6 .OMEGA..cm. If the toner concentration in the developer is decreased as by continued use, therefore, the electric charge on the electrostatic latent image carrier escapes through the carrier to disturb the latent image and spoil the developed image. If, on the other hand, the carrier retains the electric charge injected from the developing sleeve, it undesirably adheres to the image portion of the electrostatic latent image carrier. Incidentally in the so-called toner image transfer type copier, if the carrier adheres to the surface of the electrostatic latent image carrier, the carrier particles which have high rigidity inflict scratches on the carrier surface while the carrier surface is cleaned as with a blade cleaner. The developer has another disadvantage that it produces no appreciable edge effect in a sense and, therefore, fails to reproduce fine line images with sufficient sharpness. These problems arising from the insufficient electric resistance offered by the carrier can be solved to some extent by coating the carrier particles with an insulating substance such as resin. In spite of this effort, the problem of the occurrence of white streaks in developed images due to the rigidity of bristles of the magnetic brush remains. Contrary to the insufficiency of the electric resistance, the electric charge generated by triboelectrification accumulates excessively in the carrier to affect the magnitude of triboelectrical charge of the toner and shorten the service life of the developer and, in the meantime, induce the problem of adhesion of carrier particles to the non-image portion of the electrostatic latent image carrier.
The so-called binder type carrier produced in an average particle diaemter of 5-30 .mu.m by dispersing a fine magnetic powder in an insulating substance such as resin for the purpose of overcoming the drawbacks of the carrier formed of a simple magnetic substance as powdered iron has been proposed and put to actual use.
This binder type carrier has an advantage that since it is magnetized to a low level of about 1000 gausses in the magnetic field created at the developing region of the ordinary developing apparatus, it can form a magnetic brush of soft bristles and produce a developed image free from occurrence of white streaks. When a developer containing this binder type carrier is used in rapid development, a process introduced to meet the needs of the times, the developing sleeve suffers from heavy generation of heat and the developing apparatus necessitates use of a developing motor of high torque and the cost of equipment is high. Thus, the developing apparatus which uses the developer containing the aforementioned binder type carrier is generally desired to be of a type such that, by the rotation of the magnet incorporated within the developing sleeve, the bristles of a magnetic brush are rotated on the surface of the developing sleeve and the develper is transported to the developing region. When the magnet is rotated at a low rate, however, the low-frequency change of the magnetic field due to the rotation of the magnet is liable to result in uneven development and this uneven development tends to increase in proportion as the rate of development (the rate of movement of the electrostatic latent image) is increased. To prevent this uneven development, the rate of the rotation of the magnet must be increased as much as possible. Generally, in the aforementioned developing apparatus, the rate of the rotation of the magnet is set in the range of 1000 to 2500 rpm. In the case of the rapid development, the rate of the rotation must be increased proportionately to the rate of movement of the electrostatic latent image and, as an inevitable consequence, the eddy current generated within the developing sleeve is increased. As the rate of development is increased, the heat generated in the developing sleeve grows and the temperature of the developing sleeve rises and the load for rotation gains in magnitude and the torque of the motor used for rotating the magnet must be increased. Some of the electrographic systems available in the market adopt a developing apparatus of the type involving not only rapid rotation of the magnet but also supplemental rotation of the developing sleeve. Even the developing apparatus of this type is not free from the aforementioned problem which occurs during rapid development.
The method which uses a stationary magnet and requires only the developing sleeve to be rotated (hereinafter referred to as "developing sleeve rotation system") does not entail the problem ascribable to the rotation of the magnet. This fact may incite conception of an idea of precluding the occurrence of white streaks due to the aggregation of carrier particles and, at the same time, eliminating the drawbacks attendant upon the rotation of the magnet by using, in the developing sleeve rotation system, the developer containing the binder type carrier originally proposed for use in the magnet rotation system. Unfortunately, it has been experimentally ascertained that when the developer containing the binder type carrier famous for effective use in the magnet rotation system is simply diverted to the developing sleeve rotation system, the carrier adheres so heavily to the non-image portion of the surface of the electrostatic latent image carrier as to render the use of the developer infeasible. Thus, this idea has not yet been materialized.